Quasi-orthogonal space time block coding (STBC) is a method utilized in some diversity transmission systems utilized in the field of wireless communication. The appeal of quasi-orthogonal STBC is that it seeks to enable wireless communication systems to utilize advantages of diversity transmission at a transmitting station, while allowing simplified decoding techniques at a receiving station.
Diversity transmission enables one or more streams of data to be transmitted via a plurality of transmitting antennas. Diversity transmission systems are described by the number of transmitting antennas and the number of receiving antennas. For example, a diversity transmission system, which utilizes four transmitting antennas to transmit signals and a single antenna to receive signals, may be referred to as a 4×1 diversity transmission system, while a diversity transmission system, which utilizes four transmitting antennas to transmit signals and two receiving antennas to receive signals, may be referred to as a 4×2 diversity transmission system.
Each data stream may comprise a sequence of data symbols. Each data symbol comprises at least a portion of the data from the data stream. In a diversity transmission system, which utilizes orthogonal frequency division multiplexing (OFDM), each data symbol is referred to as an OFDM symbol. Each OFDM symbol may utilize a plurality of frequency carrier signals, wherein the frequencies of the carrier signals span the bandwidth of an RF channel. RF channel bandwidths may be determined, for example, based on applicable communication standards utilized in various communication systems. Exemplary RF channel bandwidths are 20 MHz and 40 MHz. One or more of the frequency carrier signals within an RF channel bandwidth may be utilized to transmit at least a portion of the data contained in the OFDM symbol. The size of each portion, as measured in bits for example, may be determined based on a constellation map. The constellation map may, in turn, be determined by a modulation type that is utilized to transport the data contained in the OFDM symbol via the RF channel.
In general, each of the data streams, which in turn comprise one or more OFDM symbols, may be referred to as a spatial stream. A diversity transmission system, which utilizes NTX transmitting antennas to transmit signals and NRX receiving antennas to receive signals, may be referred to as an NTX×NRX diversity transmission system.
In a diversity transmission system, each of the plurality of NTX transmitting antennas may transmit data symbols from a corresponding plurality of NTX space time streams. The NTX space time streams may be generated from a plurality of NSS spatial streams. Each of the data symbols in each space time stream may be referred to as a codeword. In a diversity transmission system, which utilizes quasi-orthogonal STBC, at any given time instant, each of the plurality of NTX transmitting antennas may transmit a codeword, which comprises one of the OFDM symbols, or a permutated version of the OFDM symbol, from a selected one of the NSS spatial streams.
A variation of STBC is space frequency block coding (SFBC). In a diversity transmission system, which utilizes SFBC, each codeword may comprise a subset of the frequency carriers, or tones, and corresponding data portions, in an OFDM symbol. These subsets of frequency carriers may be referred to as tone groups.
In the case of diversity transmission, with either STBC or SFBC, the transmitted signal may be modified as it travels across a communication medium to the receiving station. This signal-modifying property of the communication medium may be referred to as fading. Each of the signals transmitted by each of the plurality of transmitting antennas may experience differing amounts of fading as the signals travel through the communication medium. This variable fading characteristic may be represented by a transfer function matrix, H, which comprises a plurality of transfer function coefficients, hj, that represent the differing fading characteristics experienced by the transmitted signals.
The transmitted signals with either STBC or SFBC may be received at a receiving antenna located at a receiving station. The receiving station may process the received signals to determine estimated values for the codewords carried by the transmitted signals. However, the task of computing estimated values for the codewords may be computationally complex even when quasi-orthogonal STBC or SFBC are utilized to suppress off-diagonal crosstalk.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.